1. Field of the Invention
This invention relates to a heavy duty pneumatic radial tire. In particular, this invention relates to an improvement of service durability in the bead portion of a heavy duty pneumatic radial tire for use in an airplane wheel or the like which is forcedly subjected to large longitudinal deformation during the running under a heavy loading, for example, in the taxiing of an airplane.
2. Related Art Statement
In this type of pneumatic radial tire, a toroidal carcass ply is wound around a bead core from the inside of the tire toward the outside thereof to form a turnup portion in the bead portion, and the reinforcement of the bead portion is generally performed by embedding a stiffener, a main chafer and two auxiliary chafers in a bead portion rubber. In this case, the stiffener is disposed between the carcass ply and its turnup portion and consists of a base portion placed on the bead core and made of hard rubber stock and a thickness-reducing portion integrally united with the base portion and made of soft rubber stock. Further, the main chafer is arranged outside the carcass to extend from a lever lower than the upper end of the turnup portion along the turnup area of the carcass ply toward the inside of the tire, while the auxiliary chafer has a modulus of elasticity lower than that of the main chafer and is extended downward from a level higher than the upper end of the turnup portion so as to cover the upper end of the turnup portion and one end of the main chafer. Moreover, the two auxiliary chafers may be arranged outside the main chafer to sandwich the upper end of the turnup portion and one end of the main chafer therebetween, wherein the cords of the auxiliary chafers are crossed with each other at the same inclination angle with respect to the meridional plane of the tire.
Since longitudinal deflection of the tire is conspicuous in the taxiing of the airplane, a large deformation is forced to the bead portion. In the aforementioned conventional bead portion reinforcement, therefore, it has been confirmed that fatigue accumulates in the bead portion due to the repeated bending stress during the rotation of the tire at ground contact area and the separation failure frequently occurs in the boundary between the carcass turnup portion and the rubber surrounding therearound. Up to the present, countermeasures for solving this problem are still under examination, but a proper countermeasure has not been found.